This might be fun - and it'll work well this late in the season, and with three girls (and a strong man with a shovel... :) )
Build a sunflower playhouse. I saw this in Family Fun Magazine, and it looks really cool. I've been searching for pictures, but I can't find them. I'll keep looking.
Here are some official instructions.The idea is: plant sunflower seeds (likely unsalted. :) ) in a circle (or a square, or whatever shape you want your house to be). When the sunflowers are getting tall, rope them off to bend them down. They'll make a nice, shady canopy. It'll be about six feet tall, and sturdy. It won't blow down, either (well, except, maybe, in a hurricane). It'll be fun, because all summer the girls can come over and play in it.
With the poisoning I did in our back yard earlier this spring (still nothing will grow! :o ), I don't want to chance it with the sunflowers this season. :(
Edit:I found this link, with a picture as well!